runtime: fix leaking GC build with cores scheduler

[rdon-key]

Fix the build with -scheduler=cores -gc=leaking.

gc_stack_cores.go was included for the leaking GC, causing duplicate definitions and an undefined reference.

Limit it to GCs that perform stack scanning.

Testing

tinygo build -target=pico -scheduler=cores -gc=leaking .
tinygo build -target=pico -scheduler=cores -gc=conservative .
tinygo build -target=pico -scheduler=cores -gc=precise .

[rdon-key]

I could not find documentation listing the supported GC modes, so I used the same GC modes as gc_stack_raw.go.

[aykevl]

Thanks! Looks good to me.